For the implementation of an all fiber observation network for submarine seismic monitoring, a tri-component geophone based on Michelson interferometry is proposed and tested. A compliant cylinder-based sensor head is analyzed with finite element method and tested. The operation frequency ranges from 2 Hz to 150 Hz for acceleration detection, employing a phase generated carrier demodulation scheme, with a responsivity above 50 dB re rad/g for the whole frequency range. The transverse suppression ratio is about 30 dB. The system noise at low frequency originated mainly from the 1/f fluctuation, with an average system noise level −123.55 dB re rad/Hz ranging from 0 Hz to 500 Hz. The minimum detectable acceleration is about 2 ng/Hz, and the dynamic range is above 116 dB.

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The glass fiber honeycomb sandwich panel is an efficient structural configuration which has been widely used in several designs related to air, sea, and land transport systems. Terahertz time domain spectroscopy technology is an effective supplementary method for nondestructive evaluation of composites. Terahertz time domain spectroscopy imaging technology in the reflection configuration was employed to detect the defects of glass fiber honeycomb sandwich panels, such as a skin-core debonding and a foreign inclusion. It was shown in systematic experimental investigation that different defects were detected through terahertz imaging based on different parameters, which were obtained by comparing and analyzing the waveforms in time and/or frequency domain between normal and defective regions of the sample.

A phase-sensitive optical time-domain reflectometric (ϕ-OTDR) system based on a novel single-source dual heterodyne detection scheme is proposed and demonstrated. It uses the optical beat-frequency signals as the local oscillator signal containing the modulated frequency, frequency drift and phase fluctuation, while the signal to be detected contains all the forgoing spectral components, in addition to the vibration signal under measurement. Frequency mixing serves to isolate the pure vibration signal from the omnipresent residual frequency and phase fluctuations caused by a less strictly synchronous clock, inherent characteristics of the laser and the acousto-optical modulator, and environment temperature changes. With a reduced burden on data processing, better real-time performance is achieved as well. Using probe light pulses of 4 kHz repetition rate and 80 ns pulse width, a 9 m spatial resolution has been achieved on a 24.6 km sensing fiber, with a detectable frequency range from 5 Hz to 1.715 kHz, with a signal-to-noise ratio greater than 23.5 dB. All the above parameters are close to the maximum theoretical values. The drastically improved system demodulation characteristics foreshadow better performance and improved reliability in engineering applications.

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